Reference is hereby made to a copending application by J.A. Schuetz entitled "Resistive Overlayer for Magnetic Films" having Ser. No. 939,315, filed Dec. 8, 1986, now U.S. Pat. No. 4,857,418.
1. Field of the Invention
This invention relates to thin film devices and particularly sensors wherein a protective, resistive layer overlies at least part of a thin film conductive layer, and wherein the resistive layer is interposed between the conductive layer and an electrical contact.
2. Related Art
Certain solid state thin film devices have a conductive thin film formed directly on a substrate or formed as the top layer of a plurality of thin film layers stacked on the substrate. The thin film device operates in response to or relies upon current flowing through the conductive film. Current levels are often very small, e.g. a few milliamps. Current is supplied to the conductive film through electrical contacts formed on its surface.
Often it is a requirement that the electrical contact layer pass over the conductive thin film without making contact. Therefore an insulation layer is formed between the two. The insulating layer is usually masked and etched to define device features and provide pathways to the thin conductive film. Exposure of the conductive thin film to corrosive or even normal atmosphere during processing will result in a thin oxide layer or other highly resistive layer on the thin film's upper surface. This thin oxide layer will alter the electrical characteristics of electrical contact sites on the thin film in undesirable and often unpredictable ways. Further, since the conductive film is typically less than 1000.ANG. thick, and for many applications is less than 100.ANG. thick, it is difficult to control the etching of the insulating layer with enough precision to avoid cutting deeply into the conductive layer or even etching through it. Deep cuts in the conductive film can adversely affect current flow therein and device performance.
Protective layers overlying the conductive film could be employed, but if the protective layer is a good conductor, it could short out the electrical properties of the conductive film and reduce the already low signal current levels in the film.
Further, protective layers of a semiconductor material often diffuse into most conductive films during processing, thus adversely affecting the electrical and possibly even the mechanical properties of the conductive film.
Thus, a conductive thin film solid state device, wherein the thin film is protected during processing by a material which will not diffuse into the thin film, yet not adversely affect the electrical properties of the thin film device, is highly desirable and heretofore undisclosed.